Ultrasounds effective at detecting tumors? Are ultrasounds effective at detecting tumors? Should I be worried about cancer in my abdomen even if I have clean bloodwork and an ultrasound that showed nothing?

Very good. Ultrasound can detect tumors or masses between 2 - 3 mm, but it depends on the site and the surrounding tissues. Thyroid, pancreas, ovary, liver, etc are all organs that can be screened with ultrasound. But, you have to make sure it is a technically well performed study without any limitations.
The . The efficacy of ultrasound often depends on the area that is being imaged. Ultrasound is very poor at evaluating adrenal glands, stomach, intestines, and colon; these are usually better imaged with ct or endoscopically. However, ultrasound is excellent for evaluation of the ovaries, uterus, and gallbladder. It can also be used to detect cancer in the liver and kidneys with a fairly high degree of confidence. Assuming your primary care provider was concerned about cancer involving one of these organs, if the ultrasound was negative, your blood work was negative, and you have no other concerning symptoms, then there is no reason to worry about cancer.